The present invention relates to method and apparatus using continuous electrolytic suppression of electrolyte in eluents particularly for the analysis of anions or cations in ion chromatography.
Ion chromatography is a known technique for the analysis of ions which typically includes a chromatographic separation stage using an eluent containing an electrolyte, and an eluent suppression stage, followed by detection, typically by an electrical conductivity detector. In the chromatographic separation stage, ions of an injected sample are eluted through a separation column using an electrolyte as the eluent. In the suppression stage, electrical conductivity of the electrolyte is suppressed but not that of the separated ions so that the latter may be determined by a conductivity cell. This technique is described in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,926,559.
Suppression or stripping of the electrolyte is described in the above prior art references by a bed of ion exchange resin particles commonly referred to as a packed bed suppressor (PBS). The PBS requires periodic regeneration by flushing with an acid or base solution.
One form of packed bed suppression uses intermittent electrolytic regeneration as described in U.S. Pat Nos. 5,633,171 and 5,759,405. An electrical potential is applied through the resin in the packed bed suppressor while flowing an aqueous liquid stream to electrolyze water in the stream. For the analysis of anions, a PBS containing fully sulfonated cation exchange resin is fitted with a cathode embedded in the resin at the suppressor inlet and an anode embedded in the resin at the suppressor outlet. Hydronium ions generated at the anode displace the sodium ions which associate with the hydroxide ions for passage to waste, in this instance through the conductivity cell. This process electrochemically regenerates the suppressor, and after the electrical potential is turned off, the device can be used as a conventional PBS.
A different form of a suppressor is described and published in U.S. Pat No. 4,474,664, in which a charged ion exchange membrane in the form of a fiber or sheet is used in place of a resin bed. The sample and eluent are passed on one side of the membrane with a flowing regenerant on the other side, the membrane partitioning the regenerant from the effluent of the chromatographic separation. The membrane passes ions of the same charge as the exchangeable ions of the membrane to convert the electrolyte of the eluent to weakly ionized form, followed by detection of the ions.
Another suppression system is disclosed in U.S. Pat. No. 4,459,357. There, the effluent from a chromatographic column is passed through flow channel defined by flat membranes on both sides of the channel. On the opposite sides of both membranes are channels through which regenerant solution is passed. As with the fiber suppressor, the flat membranes pass ions of the same charge as the exchangeable ions of the membrane. An electric field is passed between electrodes on opposite sides of the effluent channel to increase the mobility of the ion exchange.
In U.S. Pat. No. 4,403,039, another form of electrodialytic suppressor is disclosed in which the ion exchange membranes are in the form of concentric tubes. One of the electrodes is at the center of the innermost tube.
Another form of suppressor is described in U.S. Pat. No. 4,999,098. In this apparatus, the suppressor includes at least one regenerant compartment and one chromatographic effluent compartment separated by an ion exchange membrane sheet. The sheet allows transmembrane passage of ions of the same charge as its exchangeable ions. Ion exchange screens are used in the regenerant and effluent compartments. Flow from the effluent compartment is directed to a detector, such as an electrical conductivity detector, for detecting the resolved ionic species. The screens provide ion exchange sites and serve to provide site to site transfer paths across the effluent flow channel so that suppression capacity is no longer limited by diffusion of ions in the bulk solution to the membrane. A sandwich suppressor is also disclosed including a second membrane sheet opposite to the first membrane sheet and defining a second regenerant compartment. Spaced electrodes are disclosed in communication with both regenerant chambers along the length of the suppressor. By applying an electrical potential across the electrodes, there is an increase in the suppression capacity of the device. The patent discloses a typical regenerant solution (acid or base) flowing in the regenerant flow channels and supplied from a regenerant delivery source. In a typical anion analysis system, sodium hydroxide is the electrolyte developing reagent and sulfuric acid is the regenerant. The patent also discloses the possibility of using water to replace the regenerant solution in the electrodialytic mode.
Another improvement in suppression is described in U.S. Pat. No. 5,248,426. This form of suppressor was introduced as a commercial product in 1992 by Dionex Corporation under the name xe2x80x9cSelf Regenerating Suppressorxe2x80x9d (SRS). A direct current power controller generates an electric field across two platinum electrodes to electrolyze water in the regenerant channels. Functionalized ion-exchange screens are present in the regenerant chambers to facilitate electric current passage with permselective ion-exchange membrane defining the chromatography eluent chamber, as in the ""098 patent. After detection, the chromatography effluent is recycled through the suppressor to form a flowing sump for electrolyte ion as well as providing the water for the electrolysis generating acid or base for suppression. Thus, no external regenerant is required and the suppressor is continuously regenerated.
In PCT Publication WO 99/11351, published Mar. 11, 1999, and incorporated herein by reference, method and apparatus are disclosed for generating an acid or base eluent in an aqueous solution and for simultaneously suppressing conductivity of the eluent in an ion exchange bed after chromatographic separation in an ion chromatography system. In one disclosed embodiment, the suppressor and eluent generator comprises: a flow-through suppressor and eluent generator bed of ion exchange resin having exchangeable ions of one charge, positive or negative, having an inlet and an outlet section in fluid communication with fluid inlet and outlet conduits, respectively; an electrode chamber disposed adjacent to said suppressor and eluent generator bed inlet section and having fluid inlet and outlet ports; a flowing aqueous liquid source in fluid communication with said electrode chamber inlet port; a first electrode disposed in said electrode chamber; a barrier separating said suppressor and eluent generator bed from said electrode chamber, the barrier preventing significant liquid flow but permitting transport of ions only of the same charge as said suppressor and eluent generator bed resin exchangeable ions; and a second electrode in electrical communication with said resin bed outlet section. The suppressor and eluent generator is used with a flow-through separator bed of ion exchange resin having exchangeable ions of opposite charge to the exchangeable ions of said suppressor and eluent generator bed, said separator bed having a sample inlet port and an effluent outlet port, said electrode chamber outlet port being in fluid communication with said separator bed inlet port, said separator bed outlet being in fluid communication with said suppressor and eluent generator bed inlet port, and a detector downstream from the generator. The aqueous liquid source can be an independent reservoir or can be a recycle conduit from the detector.
For anion analysis, one method includes (a) flowing an aqueous liquid sample stream containing anions to be detected and cation hydroxide through a separator bed of anion exchange resin with exchangeable anions to form liquid effluent including separated anions and said cation hydroxide; (b) flowing said aqueous effluent from said separator bed through a flow-through suppressor and eluent generator bed comprising cation exchange resin including exchangeable hydronium ions, so that said cation hydroxide is converted to weakly ionized form, and some of said exchangeable hydronium ions are displaced by cations from said cation hydroxide, said suppressor and eluent generator bed having inlet and outlet sections and inlet and outlet ports, liquid effluent from said suppressor and eluent generator bed flowing through said outlet port; (c) flowing an aqueous liquid through a cathode chamber proximate to said suppressor and eluent generator bed inlet section and separated by a barrier therefrom, said barrier substantially preventing liquid flow between said cathode chamber and said suppressor and eluent generator bed inlet section while providing a cation transport bridge therebetween; (d) applying an electrical potential between a cathode in said cathode chamber and an anode in electrical communication with said suppressor and eluent generator bed outlet section, whereby water is electrolyzed at said anode to generate hydronium ions to cause cations on said cation exchange resin to electromigrate toward said barrier and to be transported across said barrier toward said cathode in said cathode chamber while water in said chamber is electrolyzed to generate hydroxide ions which combine with said transported cations to form cation hydroxide in said cathode chamber; (e) flowing said cation hydroxide from said cathode chamber to the inlet of said separator column; and flowing the effluent liquid from said suppressor and eluent generator bed past a detector in which said separated anions are detected. After passing the detector, the effluent liquid can be recycled to said cathode chamber. The system can be used for cation analysis by appropriate reversal of the cation and anion functional components.
In a second disclosed embodiment of a suppressor and eluent generator bed, the second electrode is not in direct contact with the suppressor and eluent generator bed. Instead, it is adjacent the suppressor and eluent generator bed outlet section in a second electrode chamber similar to the one described above. In this embodiment, aqueous liquid exiting the detector may be recycled to the inlet of the second electrode chamber.
In a third embodiment, similar to the second one, aqueous liquid from a reservoir is pumped to the inlet of the second electrode chamber. Liquid from the outlet of the second electrode chamber is directed to the inlet of the first electrode chamber. Liquid flowing out of the first electrode chamber is directed to the inlet of the separator bed.
Publication WO 99/11351 also discloses a method of anion analysis using two electrode chambers separated from the suppressor and eluent generator bed which includes the following steps: (a) flowing an aqueous liquid sample stream containing anions to be detected and a cation hydroxide through a separator bed of anion exchange resin with exchangeable anions to form a liquid effluent including separated anions and said cation hydroxide; (b) flowing said aqueous liquid effluent from said separator bed through a flow-through suppressor and eluent generator bed comprising cation exchange resin including exchangeable hydronium ions, so that said cation hydroxide is converted to weakly ionized form, and some of said exchangeable hydronium ions are displaced by cations from said cation hydroxide, said suppressor and eluent generator bed having inlet and outlet sections and inlet and outlet ports, liquid effluent from said suppressor and eluent generator bed flowing through said outlet port; (c) flowing an aqueous liquid through an anode chamber proximate to said suppressor and eluent generator bed outlet section and separated by a first barrier therefrom, said first barrier substantially preventing liquid flow between said anode chamber and said suppressor and eluent generator bed outlet section while providing a cation transport bridge therebetween, said aqueous liquid exiting said anode chamber as an anode chamber aqueous liquid effluent; (d) flowing an aqueous liquid through a cathode chamber proximate to said suppressor and eluent generator bed inlet section and separated by a second barrier therefrom, said second barrier substantially preventing liquid flow between said cathode chamber and said suppressor and eluent generator bed inlet section while providing a cation transport bridge therebetween; (e) applying an electrical potential between an anode in said anode chamber and a cathode in said cathode chamber, whereby water is electrolyzed at said anode to generate hydronium ions which are transported across said first barrier to cause cations on said cation exchange resin to electromigrate toward said second barrier and to be transported across said second barrier toward said cathode in said cathode chamber while water in said cathode chamber is electrolyzed to generate hydroxide ions which combine with said transported cations to form cation hydroxide in said cathode chamber; (f) flowing said cation hydroxide from said cathode chamber to the inlet of said separator bed; and (g) flowing the effluent from said suppressor and eluent generator bed past a detector in which said separated anions are detected.
The anode chamber aqueous liquid effluent may be recycled through said cathode chamber. Alternatively, after detection in step (g), the suppressor and eluent generator bed effluent may be recycled through said anode chamber.
In PCT Publication WO 99/44054, published Sep. 2, 1999, and incorporated herein by reference, method and apparatus are disclosed for continuously electrolytically suppressing the conductivity of an eluent in an ion exchange bed previously used in separating ions in a separator bed.
Referring first to the apparatus, the suppressor includes (a) a flow-through suppressor bed of ion exchange resin having exchangeable ions of one charge, positive or negative, having a liquid sample inlet and an outlet section in fluid communication with suppressor inlet and outlet ports, respectively, (b) a first electrode chamber disposed adjacent to said suppressor inlet section and having fluid inlet and outlet ports, (c) a first electrode disposed in said first electrode chamber, (d) a barrier separating said suppressor bed from said first electrode chamber, said barrier preventing significant liquid flow but permitting transport of ions only of the same charge as said suppressor bed resin exchangeable ions, (e) a second electrode in electrical communication with said resin bed outlet section, and (f) a recycle conduit providing fluid communication between said suppressor outlet port and said electrode chamber inlet port.
Opposite faces of the barrier are in electrical communication with the first and second electrodes, respectively, in direct contact or through conductive medium. For example, the second electrode is in electrical communication with the barrier through the conductive suppressor bed.
The suppressor is normally used in combination with (g) a flow-through separator bed of ion exchange resin having exchangeable ions of opposite charge to the exchangeable ions of said suppressor bed, said separator bed having a sample inlet port and an outlet port, said separator bed outlet port being in fluid communication with said suppressor bed inlet port, and with a detector disposed in the path of said recycle conduit to detect sample flowing through said conduit.
In one embodiment, the second electrode is disposed in contact with said ion exchange resin in said suppressor outlet section. In another embodiment, the suppressor combination includes (h) a second electrode chamber disposed adjacent to said suppressor outlet section and having fluid inlet and outlet ports, and (i) a second barrier separating said suppressor bed from said second electrode chamber, said barrier preventing significant liquid flow but permitting transport of ions only of the same charge as said suppressor bed resin exchangeable ions, said second electrode being disposed in said second electrode chamber.
For anion analysis, the suppressor bed ion exchange resin is a cation exchange resin, the first electrode is a cathode, and the second electrode is an anode. The opposite polarities apply for cation analysis.
Referring to one embodiment of the method, anion analysis is performed by the following steps: (a) flowing an aqueous liquid sample stream containing anions to be detected and cation hydroxide through a separator bed of anion exchange resin with exchangeable anions to form liquid effluent including separated sample anions and said cation hydroxide,(b)flowing said aqueous effluent from said separator bed through a flow-through suppressor and comprising cation exchange resin including exchangeable hydronium ions, so that said cation hydroxide is converted to weakly ionized form, and some of said exchangeable hydronium ions are displaced by cations from said cation hydroxide, said suppressor bed having inlet and outlet sections and inlet and outlet ports, liquid effluent from said suppressor bed flowing through said outlet port,(c) flowing the effluent liquid from said suppressor past a detector in which said separated sample anions are detected, (d) recycling said liquid effluent from said detector through a cathode chamber proximate to said suppressor bed inlet section and separated by a first barrier therefrom, said first barrier substantially preventing liquid flow between said cathode chamber and said suppressor bed inlet section while providing a cation transport bridge therebetween, and (e) applying an electrical potential between a cathode in said cathode chamber and an anode in electrical communication with said suppressor bed outlet section, whereby water is electrolyzed at said anode to generate hydronium ions to cause cations on said cation exchange resin to electromigrate toward said barrier and to be transported across said barrier toward said cathode in said cathode chamber while water in said cathode chamber is electrolyzed to generate hydroxide ions which combine with said transported cations to form cation hydroxide in said cathode chamber. In another embodiment, the liquid effluent is recycled through an anode chamber proximate to said suppressor bed outlet section and separated by a barrier of the same type as the first barrier. The anode is disposed in the anode chamber.